The object of the present invention is an anti-bounce device for a mass striking a target element and generating seismic impulses.
The device according to the invention is particularly useful for achieving impact seismic sources which generate in the ground seismic waves through the percussion of a moving mass against a target element which it is hurled at by the effect of gravity and/or motive means. A percussion source for seismic prospecting on land comprising a mass falling along a guiding element towards a target element anchored against the ground surface is described in French Patent No. 2,398,316. Another percussion source, more particularly adapted for being used in a well bore, is described in French Patent No. 2,552,553. The use of mot or means for throwing a mass against a target element anchored in a well is described in French Patent No. 2,558,601.
When the mass drops freely, it generally bounces after its impact against the associated target element and it then comes back and strikes it once or several times with decreasing strengths. The "signature" of the seismic source, i.e the strength of the emitted impulses, thus comprises in this case several secondary impulses of decreasing amplitudes following the main impulse and this has the effect of disturbing the seismic recordings corresponding to the seismic reflections of the emitted impact pulse on the subsoil discontinuities.
Through French Patent No. 2,509,052, a device is known which avoids the multiple shocks on a target element, connected with the ground surface of a mass guided in its fall by guiding means, these multiple shocks being due to the bounces of the mass after its first impact. This device essentially comprises a deformable element fastened to the guiding means and control means adapted for applying the deformable element against the side wall of the mass after its first bounce thereby immobilizing it before it falls down again. The control means comprise, for example, a mobile element which is radially moved by the action of a hydraulic jack and an impact detector delivering a control signal resulting in the tripping of the jack.
This device very efficiently blocks any further fall of the mass but, due to its transverse position perpendicular to the longitudinal axis of the guiding means, its space requirement would not be compatible with the generally limited dimensions of the drill holes where the well seismic sources are taken down.
Through published French Patent Application 2.631.458 an anti-bounce device is known which is well-adapted to the cases where the lateral space around the direction of fall is limited, and which is characterized by the use of electromagnetic means in order to exert an intermittent attractive face on the mass. As a result the mass sticks to the target element that it has just struck at the end of the fall. Such a device is suitable to any application where it is possible to use electromagnetic means and to connect them with feeding cables to a electric source.